Solar array inverter with maximum power tracking

ABSTRACT

An inverter for use in connecting a DC power source to the utility grid includes a single DC-AC conversion stage, maximum (source) power tracking, and current control based on feed-forward compensation as a function of input power voltage error, rectified utility line voltage, and a scaled inverse of RMS utility line voltage. Various topologies of the inverter power stage are developed for bi-directional power-flow operation after those developed for unidirectional power-flow operation. Various embodiments also include over-voltage protection, over-current protection, under-voltage protection, over-temperature protection, and stand-by battery with battery management control, while still others are adapted for a multiple-channel front-end distributed power system with distributed maximum power tracking for serving as a single DC power source input to the inverter system downstream with controllers, emergency or auxiliary loads, and alternative current feedback control systems. Improved control architectures are developed with bipolar sensing of the inverter output current and a bipolar sinusoidal reference signal.

FIELD OF THE INVENTION

The present invention relates to electrical power supply and interconnection systems. More specifically, the present invention relates to systems and methods for connecting the electrical output of solar arrays to the utility power grid.

BACKGROUND

Typically, DC-to-AC power conversion from a solar-array source to the utility grid is achieved in two power converter stages to transfer the maximum available power from the solar array and supply an in-phase AC current into the utility grid. See, for example, U.S. Pat. Nos. 6,281,485 and 6,369,462; and D. K. Decker, “Method for Utilizing Maximum Power from a Solar Array,” JPL Quarterly Technical Review, 1972, Vol. 2, No. 1, pages 37-48. One power stage is controlled to track the array peak power, and the other power stage controlled to deliver the AC current to the grid. This typical configuration leads to higher component counts and possibly reduced power conversion efficiency. One benefit of the two stages of power conversion is simplicity of the control design, since the two control loops are decoupled. One control loop tracks the peak-power array voltage by controlling a first converter that is interfaced between the solar-array source and the second converter, while the other control loop produces the rectified sinusoidal current being fed from the second converter to the utility grid through a switching bridge topology that is synchronized to the utility frequency. Such decoupled pairs of control loops are described, for example, in “Sequentially-Controlled Distributed Solar-Array Power System with Maximum Power Tracking” by Kasemsan Siri and Kenneth A. Conner, presented at the Aerospace Conference, Mar. 6-12, 2004, Big Sky, Mont.

The output voltage of the first converter, which becomes the input voltage of the second converter, does not need to be so well regulated and may have significant voltage ripple depending on the size of the output filter capacitor and the operating power level. Therefore, the two cascaded-power-stage configuration provides design flexibility in choices of output voltage level and size of the output filter capacitance, as well as the input filter capacitance of the first power stage. For applications that require low operating solar-array voltage (less than 100V) for safety reasons, the two-cascaded-stage configuration provides more design freedom to choose economical power components, especially the input and output filter capacitors. In this configuration, the fundamental frequency of the solar-array AC voltage ripple can be selectively controlled to be different from (or not synchronized with) the multiples of the utility frequency. Power conversion efficiency of 85% to 90% is achieved in some cascaded-power-stage designs.

There is thus a need for further contributions and improvements to DC-to-AC power conversion technology.

SUMMARY

When power conversion efficiency greater than 90% is required, switch-mode DC-to-AC power conversion in one power stage is preferred, though it comes with some sacrifice of design freedom in choices of input filter capacitors and more complex control loops that are not independent from one another. Some embodiments of the present invention employ a novel control approach and power architecture that process power transferred from the DC source (such as a solar array) to the AC utility grid in a single stage of power processing (one DC-DC converter). In this approach, the input filter capacitance across the solar array source is preferably selected to ensure the acceptable AC array-voltage ripple as well as robust stability of the array voltage and the in-phase AC current delivered to the utility grid.

It is an object of some embodiments of the present invention to supply regulated sinusoidal AC current to the utility grid from a solar array or other DC power source with maximum power tracking in a single stage of DC-DC power conversion. It is an object of other embodiments to maximally utilize the available power of the array source as load demand increases above the peak power of the DC source. It is an object of still other embodiments to reduce power dissipation from a single stage of power conversion, while in others an object is to provide flexibility between stand-alone and grid-tie modes of inverter operation.

An object of some other embodiments is to allow power expansion capability through a parallel connection of commercial, off-the-shelf (COTS) DC-DC converters. In other embodiments, an object is to provide an inverter that may be used with a variety of renewable energy sources, such as fuel cells and wind generators. An object of still other embodiments is to prevent a catastrophic short-circuit due to an inadvertent loss of grid-voltage synchronization.

These objects and others are achieved by various forms of the present invention. One form of the present invention provides maximum power tracking and voltage regulation at the output of a solar array, which output is filtered and passed through a DC-DC converter. The DC-DC converter is controlled using one or more control strategies described herein, including feed-forward compensation as a function of array output voltage error, scaled converter output voltage, and the scaled inverse (or square of the scaled inverse) of the RMS value of the output voltage. In preferred forms, systems according to the present invention include over-voltage protection, over-current protection, under-voltage protection, and over-temperature protection as part of the feed-forward compensation loop. In still other forms, emergency or auxiliary loads are switchable into the system, while in still other forms the inverter system is adapted to handle multiple input sources and/or battery back-up.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block/schematic diagram of a maximum power tracking solar inverter according to a first embodiment of the present invention.

FIG. 2 is a block/schematic diagram of a second embodiment of solar power inverter, with over-voltage, under-voltage, over-current, and over-temperature protection.

FIG. 3 is a block/schematic diagram of a third embodiment of solar power inverter according to the present invention for use with a multi-channel solar array.

FIG. 4 is a block diagram of a solar inverter with maximum-power tracking and battery back-up.

FIG. 5 is a block diagram of an over-current protection controller for use in various embodiments with the present invention.

FIG. 6 is a block diagram of an over-voltage protection controller for use in various embodiments with the present invention.

FIG. 7 is a schematic diagram of a 50/60 Hz switching bridge circuit for use in various embodiments of the present invention.

FIG. 8 is a schematic diagram of a 50/60 Hz bridge driver circuit for use in various embodiments of the present invention.

FIG. 9 is a schematic diagram of a combined DC-DC converter and switching bridge circuit for use in various embodiments of the present invention.

FIG. 10 is a schematic diagram of a push-pull DC-DC converter combined with a 50/60 Hz unfolding network for use in various embodiments of the present invention.

FIG. 11 is a graph illustrating reference current and output current in an average output current regulation mode of a DC-DC converter in various embodiments of the present invention.

FIG. 12 is a graph illustrating reference current and output current in a peak-current programmed control mode of a DC-DC converter in various embodiments of the present invention.

FIG. 13 is a graph illustrating reference current, upper threshold, lower threshold, and output current in a hysteretic current regulation mode of a DC-DC converter in various embodiments of the present invention.

FIG. 14 is a graph illustrating reference input current and output current in a continuous conduction mode (with zero-current turn-on) of a DC-DC converter in various embodiments of the present invention.

FIG. 15 is a graph of current-voltage and power-voltage characteristic curves for solar arrays in modes I and II according to certain embodiments.

FIG. 16 is a graph of solar array output power (I_(SA)·V_(SA)) in modes I and II according to certain simulated embodiments.

FIG. 17 is a graph of utility grid current I_(AC) and voltage V_(UT) over time according to one simulated embodiment of the present invention.

FIG. 18 is a graph of utility grid current I_(AC) and solar array voltage V_(SA) over time in one simulated embodiment of the present invention.

FIG. 19 is a graph of current output I_(OUT) and voltage output V_(UT) from the DC-DC converter over time in one simulated embodiment of the present invention.

FIG. 20 is a graph of output current I_(OUT) and normalized reference current I_(REF) over time according to one simulated embodiment of the present invention.

FIG. 21 is a graph of solar array output power (I_(SA)·V_(SA)) and MPT set-point voltage V_(SP) over time according to one simulated embodiment of the present invention.

FIG. 22 is a graph of solar array output voltage V_(SA) and voltage error amplifier signal V_(ERR) over time according to one simulated embodiment of the present invention.

FIG. 23 is a graph of the step response of solar array power I_(SA)·V_(SA), solar array voltage V_(SA), and utility grid voltage V_(UT) over time according to one simulated embodiment of the present invention.

FIG. 24 is a schematic diagram of a combined DC-DC converter and switching bridge circuit for use in various embodiments of the present invention.

FIG. 25 is a block/schematic diagram of a maximum power tracking solar inverter with a stand-by battery and emergency stand-alone load according to a second embodiment of the present invention.

FIG. 26 is a schematic diagram of an inverter that allows bi-directional flow while driving inductive loads, for use with some embodiments of the present invention.

FIG. 27 is a waveform diagram of MOSFET gate-to-source driving signals for use in conjunction with the control architecture shown in FIG. 28.

FIG. 28 is a block/schematic diagram of a maximum power tracking solar inverter with a stand-by battery and emergency stand-alone load according to a third embodiment of the present invention.

FIG. 29 schematic diagram of an alternative inverter design for use with some embodiments of the present invention.

FIG. 30 is a waveform diagram of MOSFET gate-to-source driving signals for use in conjunction with the control architecture shown in FIG. 28 and the inverter shown in FIG. 29.

FIG. 31 is a block/schematic diagram of a maximum power tracking solar inverter using an isolated bi-directional inverter according to a fourth embodiment of the present invention.

FIG. 32 is a schematic diagram of another alternative inverter design that provides for bi-directional power flow in some embodiments of the present invention.

FIG. 33 is a schematic diagram of yet another alternative inverter design that provides for bi-directional power flow in some embodiments of the present invention.

FIG. 34 is a schematic diagram of still another alternative inverter design that provides for bi-directional power flow in some embodiments of the present invention.

FIG. 35 is a waveform diagram of MOSFET gate-to-source driving signals for use in conjunction with the control architecture shown in FIG. 28 and the inverter shown in FIG. 34.

FIG. 36 is a waveform diagram of alternative MOSFET gate-to-source driving signals for use in conjunction with the control architecture shown in FIG. 31 and the inverter shown in FIG. 34.

FIG. 37 is a schematic diagram of yet another alternative replacement for the bi-directional power-flow inverter topologies with front-end push-pull or front-end full-bridge configurations as shown in FIG. 34.

FIG. 38 is a block diagram of a maximum power tracking control circuit used with various embodiments of the present invention.

FIGS. 39 and 40 are alternative analog differentiator circuits for use with the MPT control circuit of FIG. 38.

FIG. 41 is a graph of converter input current I_(IN), DC power source current and voltage I_(SA) and V_(SA), and power signal of a non-linear DC source P_(SA) that are used in some embodiments of the present invention.

FIG. 42 is a graph over time of array output voltage and power signals, and the associated array power/voltage characteristic curve, when some embodiments of the present invention operate at the peak power point of the array.

FIG. 43 is a graph over time of array output voltage and power signals, and the associated array power/voltage characteristic curve, when some embodiments of the present invention operate away from the peak power point of the array.

FIG. 44 is a block diagram of a maximum power tracking control circuit with additional dither signal injection.

DESCRIPTION

For the purpose of promoting an understanding of the principles of the present invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will, nevertheless, be understood that no limitation of the scope of the invention is thereby intended; any alterations and further modifications of the described or illustrated embodiments, and any further applications of the principles of the invention as illustrated therein are contemplated as would normally occur to one skilled in the art to which the invention relates.

1. Overall System Description

Generally, system 100 adapts the output of a solar array (in this example) or other DC power source for use with the utility power grid, maintaining approximately maximum power output from the source and efficiently transferring that power to the grid. Solar array voltage and power are regulated and converter output current and voltage are controlled. Various exemplary embodiments are described herein, including systems adapted for over-voltage protection, over-current protection, under-voltage protection, over-temperature protection, battery back-up, and multiple inputs. Relevant aspects from these embodiments may be used individually or combined to suit a given situation, as will be understood by those of ordinary skill in the art given the disclosure herein.

Turning to FIG. 1, system 100 adapts the output of solar array 110 for connection to a utility grid at terminals 180. Generally, power flow proceeds along components at the top of FIG. 1, while measurement and control signals and components are shown below the top row of components in the figure. As will be understood by those of ordinary skill in the art, the output current I_(SA) and output voltage V_(SA) of solar array 110 are measured for use at maximum power tracking (MPT) control 120 and solar array voltage regulation error amplifier 130, then filtered by capacitor C_(BULK) and line filter 105. To provide input for current-mode DC-DC converter circuit 140. Feedback and control are provided to converter circuit 140 through the voltage regulation error amplifier 130, compensated current reference generator 150, and over-voltage protection circuit 160. Further details for these subsystems are provided herein. In addition to feedback signals for control purposes, current-mode 140 provides its power output to bridge circuit 170, which supplies the grid through terminals 180.

Employing solar-array voltage V_(SA) and solar-array current I_(SA), as the feedback signals, the MPT control subsystem 120 continuously updates the set-point voltage reference V_(SP). This set-point voltage reference commands the solar-array voltage regulation subsystem (or SA voltage regulation error amplifier) 130 to produce the error voltage drive signal V_(ERR) for regulation of the solar-array voltage, V_(SA), at the level corresponding to V_(SP). Upon reaching a steady state of operation, V_(SA) is controlled to swing back and forth around the peak-power solar array voltage with acceptable ripple voltage at the frequency twice that of the utility frequency. The V_(ERR) signal, which serves as an input-power commanding voltage and is sufficiently band-limited to have negligible AC ripple voltage (especially at twice the utility frequency), commands the compensated current reference generator subsystem 150 to control the converter power stage 140 to properly deliver the output current of the rectified sinusoidal wave, I_(OUT).

Subsequently, the DC-AC switching bridge inverter subsystem 170 provides switching control from bridge driver 174 at the utility frequency to the 50/60 Hz switching bridge 172 that commutates I_(OUT) into an AC current in synchronization with the AC utility voltage. Switching bridge 172 delivers the in-phase AC current I_(AC) to the utility grid through output filter 176 with low total-harmonics distortion, as considered by Songquan Deng, Hong Mao, J. Mazumdar, I. Batarseh, K. K. Islam, “A New Control Scheme for High-Frequency Link Inverter Design,” Applied Power Electronics Conference and Exposition, APEC Eighteenth Annual IEEE, Feb. 9-13 2003, Vol. 1, pages 512-517. When an over-voltage condition is detected from the rectified utility voltage V_(UT), the over-voltage protection subsystem 160 further reduces the error voltage drive signal V_(ERR) through an active pull-down transistor Q. Under this condition, either the absolute average of V_(UT) is regulated at a predetermined level or the power converter is shut down while the absolute average of V_(UT) continues to exceed a predetermined level. For detailed discussions of these strategies, see Kasemsan Siri, Vahe A. Caliskan, C. Q. Lee, “Maximum Power Tracking in Parallel-Connected Converter Systems,” IEEE Transactions on Aerospace and Electronics Systems, July 1993, Vol. 29, pages 935-945; and Kasemsan Siri and Kenneth A. Conner, “Fault-Tolerant Scaleable Solar Power Bus Architectures with Maximum Power Tracking,” Proceedings of the Sixteenth Annual IEEE APEC, 2001, Vol. 2, pages 1009-1014.

The average-current regulation subsystem (including converter circuit 140 and compensated current reference generator 150) is part of the innermost control loop that requires the fastest control dynamics (or the highest control unity-gain bandwidth) as compared to those of the array-voltage regulation subsystem (120, 130) since the system preferably minimizes harmonic distortion of the rectified sinusoidal current output I_(OUT). See Hong Mao, Jaber Abu-Qahouq, Shiguo Luo, Issa Batarseh, “New Zero-Voltage-Switching Half-Bridge DC-DC Converter and PWM Control Method,” Eighteenth Annual IEEE Applied Power Electronics Conference and Exposition, APEC February 2003, Vol. 2, pages 635-640; and Jaber Abu-Qahouq and Issa Batarseh, “Unified Steady-State Analysis of Soft-Switching DC-DC Converters,” IEEE Transactions on Power Electronics, September 2002, Vol. 17, No. 5, pages 684-691. Serving as the commanding reference-current signal for the average current regulation subsystem 144, I_(REF) possesses the rectified sinusoidal waveform that is properly scaled from the rectified utility voltage V_(UT).

To significantly reduce the variation of the error voltage V_(ERR) at an array power level within a utility voltage range, a feed-forward compensation technique is utilized to continuously update I_(REF) through the feed-forward compensated multiplier 152 from which the output reference current signal I_(REF) is the scaled product of three quantities: V_(ERR), the instantaneous value of V_(UT) (through scaler 158, which multiplies V_(UT) by k₁), and the inverse of the RMS squared value of V_(UT) (i.e., V_(UT) having passed through RMS calculation and scaler 154 and squarer 156).

To provide low total-harmonic distortion in the in-phase AC current I_(AC), V_(ERR) should have negligibly small AC ripple content superimposed on its DC operating point. Therefore, the control loop gain frequency response of the solar-array regulation subsystem needs to have a relatively low unity-gain bandwidth (such as a 10 Hz to 15 Hz bandwidth). Consequently, the array-voltage regulation control loop is preferably designed to be insensitive to the fundamental frequency of the array-voltage AC ripple (at 100/120 Hz) but only to the DC or much lower frequency components of the solar array voltage. See Kasemsan Siri, “Study of System Instability in Current-Mode Converter Power Systems Operating in Solar Array Voltage Regulation Mode,” APEC, New Orleans, La., Feb. 6-10, 2000, Vol. 1, pages 228-234; and U.S. Pat. No. 6,009,000 to Kasemsan Siri (“Shared-Bus Current Sharing Parallel Connected Current-Mode DC to DC Converters,” issued Dec. 28, 1999). Since the solar array source does not have stiff characteristics of a voltage source, the system requires a proper value of the input filter capacitance C_(BULK) to limit the array voltage ripple (at 100/120 Hz) to an acceptable amplitude (of 5% or less) relative to the array operating voltage. See R. Ramakumar, I. Abouzahr, K. Krishnan, K. Ashenayi, “Twenty-five Design Scenarios for Integrated Renewable Energy Systems, Energy Conversion,” IEEE Transactions, December 1995, Vol. 10, Issue 4, pages 736-746; and R. Matson and R. McConnell, “DOE/NREL Fundamental and Exploratory Research Into Solar Cells,” Photovoltaic Specialists Conference Record of the Twenty-Ninth IEEE, May 19-24, 2002, pages 1343-1347 for discussion of this design issue.

The compensated current reference generator 150 in this embodiment includes a root mean-square (RMS) extraction and scaling circuit 154, signal squarer 156, and a three-input multiplier 152. The RMS and scaling circuit 154 extracts a low-pass filtered signal that has a DC component proportional to the RMS value of the utility line voltage V_(UT). The scaled RMS signal preferably has negligible AC components to ensure low harmonic content of the controlled sinusoidal current supplied to the utility grid. The scaled RMS signal is squared by squarer 156, then inverted after being fed to the Z input of the three-input multiplier circuit 152. The inverse squared RMS signal is used as a feed-forward compensation factor, 1/Z, for the three-input multiplier that senses, at the input Y, a scaled (by factor k₁ at multiplier 158), rectified utility voltage signal having a rectified sinusoidal wave shape, and multiplies the V_(ERR) signal as input X by the product of the sensed rectified line-voltage signal (Y) and the feed-forward compensation factor (1/Z). The multiplier outputs the computed result (X*Y/Z) as the reference current signal, I_(REF), that commands the average current controller 144 in the innermost control loop to regulate the output current of converter 142 accordingly.

The line-filter 105 interfacing to the array source 110 provides sufficient differential-mode attenuation of line voltage and current ripple at medium and high frequencies as well as common-mode attenuation of the conducted-emission at high frequencies, as discussed in Ramakumar et al., supra. So does the output filter 176, which interfaces to the utility grid. The 50/60 Hz switching bridge 172 is driven by a set of switching signals derived from the utility voltage through the bridge driver circuit 176 such that the resulting AC current I_(AC) that is converted from the converter 142 output current '_(O)ur is always in-phase with the utility voltage.

In some embodiments, transient energy absorption devices are included within either the solar array 110 or the line filter 105 to provide transient voltage suppression from lightning and other induced transients that can be coupled into the solar array and the transmission line being connected between the solar array and the line filter. In other embodiments, the converter 140 output and the DC-to-AC switching bridge inverter subsystem 170 are electrically isolated from the solar array and all the control circuits (120, 130, 144, 150, and 160) by exploiting an isolated-output DC-DC converter 142 and isolated current and voltage sensing schemes for both converter output current I_(OUT) and output voltage V_(UT).

The basic solar inverter power system can be extended to different architecture and control configurations as shown in FIGS. 2, 3, and 4, where the inverter system controller independently provides the reference sinusoidal signal that can be synchronized with the utility frequency. In this manner, the feed-forward compensation does not need to perform the squarer function to the feed-forward utility voltage. The feed-forward compensation design is therefore simpler, where 1/x is used instead of 1/x² to perform feed-forward compensation (as x represents the utility voltage feed-forward signal V_(UT)). As shown in FIG. 2, multiplier with feed-forward compensator 252 provides the product I_(REF) of its three inputs. In this case, the first input is again the amplified input voltage error signal V_(ERR), which is drawn low via pull-down transistor Q if failure conditions are detected by over-current protection circuit 282, over-temperature protection circuit 284, or over-voltage protection circuit 286. These protection circuits provide low-voltage output during a failure through their respective diodes D1, D2, and D3 to pull the gate of transistor Q low, which brings the “X” input of multiplier 252 to a low or zero value.

The second input to multiplier 252, which input has a rectified sinusoidal waveform, is drawn from free-running sinusoidal wave form generator 292, scaled by factor K₁ by scaler 158. As discussed just above, the third (Z) input to multiplier 252 is the RMS utility voltage calculated by RMS unit 154, as inverted by inverter 256. The product, I_(REF)) provides reference current to average current controller 144, as discussed above, in relation to FIG. 1.

Free-running sinusoidal reference generator 292 is, in some embodiments, an analog-filter sinusoidal waveform generator circuit that accepts a square-wave signal input and produces the in-phase sinusoidal output at the same frequency of the square-wave input, as will be understood by those skilled in the art. The square-wave input always has a fixed amplitude and is derived from the AC utility grid voltage through a zero-crossing detector circuit that detects zero-voltage crossing of the utility grid voltage without being sensitive to the amplitude of the utility grid voltage, but only to the timing of the utility grid voltage zero-crossings. The zero-voltage crossing detector is preferably a voltage comparator that provides a bipolar output voltage waveform of fixed amplitude.

Free-running sinusoidal generator 292 also provides a square-wave control input to bridge driver 174, which again controls the switching bridge 172. The output of switching bridge 172 passes through output filter 176 to relay switching network 296. Relay switching network 296 chooses between outputs to the grid via terminals 280 and the emergency standalone load 298 based on the output control signal G from utility voltage drop-out detector 294. The control signal G delivers one of two possible control states: grid-tie state or stand-alone state. Design and construction of these components is within the capability of one of the ordinary skill in the art based on the present disclosure, and will not be described further herein.

FIG. 3 provides the solar inverter power system that regulates its DC input voltage to a corresponding set-point voltage V_(SP), selected by voltage source 325. MPT control (as in subsystem 120 of FIG. 1) is unnecessary in this embodiment since the upstream distributed solar array power systems 310 already achieve MPT through their individual MPT controllers. The remainder of the embodiments shown in FIG. 3 operate substantially as discussed above in relation to the embodiments shown in FIG. 2.

FIG. 25 shows an inverter system similar to that in FIG. 2, but with the addition of a stand-by battery to improve performance during high transient surges in the power demanded by the stand-alone load that sometimes require a start-up current significantly above the steady-stage current. In some applications of the embodiment shown in FIG. 2, the system has not been able to bring the stand-alone load to its typical steady state because the start-up load power was much higher than the array peak power, subsequently leading to a lock-up state wherein the load RMS voltage is too low for proper operation. The stand-by battery added in the embodiment shown in FIG. 25 has active controlled switches Q_(X) and Q_(Y) providing a solution that allows high power surges during the start-up of the stand-alone load to quickly bring the load voltage and power to reach its steady-state operating condition. In the embodiment being discussed, Q_(X) and Q_(Y) are N-channel enhancement MOSFETs that have built-in body diodes. Other switches are used in alternative embodiments, as would be understood by those skilled in the art.

Electrical isolation between the converter output and the remaining control circuits, including the solar array, the line-filter, the stand-by battery, and the converter input, is achievable through isolation schemes within the following subsystems shown in FIGS. 2, 3, and 25: (1) DC-DC converter 142 through its internal isolation transformer, (2) bridge driver circuit 174 through its isolated driving transformer or opto-isolated driving circuit, (3) utility voltage dropout detector 194 through its voltage-sense transformer, (4) the sensed converter output current I_(OUT) through an isolated current sense scheme such as a current-sense transformer, and (5) the sensed converter output voltage V_(UT) through an isolated voltage sensing scheme such as an opto-isolated voltage sensing circuit.

The battery management control circuit 218 detects the under-voltage condition of the load voltage V_(UT) when the system operates in stand-alone mode. Such an under-voltage condition occurs as a consequence of a high start-up load current, causing the control circuit to disable MPT controller 120 by changing the MPT_Enable signal to be inactive for a predetermined duration. The inactive state of MPT_Enable immediately resets the commanding set-point V_(SP) to its preset minimum level, allowing the DC-DC converter 142 to absorb more input current to which both the stand-by battery and the array source can contribute. Once the load demand reaches steady state, the transient under-voltage condition disappears and the MPT controller 120 resumes its active function to bring the array voltage V_(SA) into at least its peak power voltage. During steady state of the stand-alone mode of operation, the healthy solar array voltage is typically greater than the battery voltage, so MOSFETs Q_(X) and Q_(Y) are turned off, resulting in no power drain from the stand-by battery. During the absence of sunlight when the inverter is in stand-alone mode, the array source is inactive, subsequently causing MPT_Enable signal to be inactive, thereby disabling MPT controller 120 and setting V_(SP) to the minimum. At the same time, MOSFET Q_(Y) is turned on to allow the stand-by battery to provide power to the stand-alone load due to the natural conduction of the body diode within MOSFET Q_(X). During the period in which the MPT controller 120 is disabled, the load voltage V_(UT) may increase until it reaches the over-voltage protection threshold, where the OVP and circuit 286 will actively regulate the RMS output voltage of the converter power stage 142 to ensure a safe operating condition.

Once the period expires in which MPT operation is disabled, MPT operation is resumed while the battery is still in stand-by mode (MOSFET Q_(X) is off and MOSFET Q_(Y) is on). Therefore, the MPT commanding set-point voltage V_(SP) increases from its minimum value, and consequently the converter input voltage increases toward the maximum power voltage. Due to the reverse bias across the body diode within MOSFET Q_(X) while Q_(Y) may remain turned on, the stand-by battery 214 will stop transferring its power. Nonetheless, both MOSFETs Q_(X) and Q_(Y) may be turned off to ensure battery energy conservation. During steady-state operation of stand-alone load or grid tie, the stand-by battery 214 may be charged at a slow rate, while Q_(Y) is turned off, by controlling Q_(X) to be in a linear region of MOSFETs characteristics as summarized in Table. For a much faster rate of battery charging, Q_(X) can be turned on continuously or operated in a linear PWM mode that allows the charging current to be regulated by the linear control of the Q_(X) MOSFET which is periodically turned off to control thermal stresses around Q_(X) and maintain the battery in a safe operating condition. Furthermore, during an inverter shutdown mode of operation, the stand-by battery 214 may be charged at the faster rate to quickly restore its healthy state of charge.

TABLE State State Battery State State of MPT of Array Mode of Q_(x) of Q_(y) Inverter Mode Controller Source Benefits Isolation OFF OFF Shut-down Stand-by Inactive Conserve battery GT or STA Active Active energy Stand-By OFF ON STA Active Active For V_(SA) > V_(BAT), transfer array peak power to STA load & conserve battery STA Disable Inactive Battery power for load or weak voltage regulation, MPT set V_(SP) = minimum Charging Linear OFF GT or STA Active Active Maintain healthy state Regulated of charge at low power while tracking array peak power to grid or stand- alone load Input ON or OFF Shut-Down Stand-by Active Fast battery charging to Voltage Linear restore healthy state of Clamping PWM charge No power transfer to inverter Battery ON ON STA Disable Inactive Low-loss power transfer Dominated to stand-alone load with Input voltage regulation, MPT set V_(SP) = minimum ON or ON Shut-Down Stand-by Active Fast battery charging to Linear restore healthy state of PWM charge No power transfer to inverter

FIG. 4 provides a system level application interconnected configuration that allows either grid-tie or stand-alone operation to take place through a relay-switching network 496 interfacing between the utility grid 480 and the inverter output line filter. Through the front-end relay-switching network 406 interfacing between the solar array 410, the back-up battery 414, and the power input port of DC-DC converter 442, the inverter system also autonomously operates in either normal MPT or battery back-up mode depending on the health status of the utility grid voltage and the inverter output voltage/current. When the utility grid is in either an over- or under-voltage condition, the system transfers the solar power to the stand-alone load (or auxiliary load) 498 instead. In this stand-alone mode, the reference sinusoidal signal from reference generator 492 does not need to be synchronized with the utility grid voltage, and neither does the inverter output voltage across the stand-alone load 498, which in most cases serves emergency purposes such as emergency lighting. As shown in FIG. 4, a battery charger 416 is part of an optional back-up subsystem 404 that can be added to the basic module 402 if superior battery reliability is desired. This exemplary architecture offers the full utilization of the single stage of power conversion (DC-DC converter 142) for all modes of operation. The DC-DC converter 442 can also consist of either a single module or parallel-connected converter modules that allows for expansion for higher power capability. Under/over inverter output voltage (UVP & OVP) protection, inverter output over-current protection (OCP), and inverter over-temperature protection (OTP) can be inserted at 480 similarly to that protection shown in FIG. 3.

Through computer simulations discussed below, a new control approach for a solar-based inverter that tracks the maximum available power and produces a near-unity power factor was validated. It is shown that the systems described herein produce excellent signal-to-noise ratios for the feedback signals, ensuring reliable and robust tracking of maximum power while tightly regulating the sinusoidal waveform of AC current supplied to the utility grid with almost unity power factor. It is also shown that using feed-forward compensation coupled with single-stage DC-DC conversion, the proposed inverter system is simplified when compared to many conventional inverter designs. This is still achieved while maximum power is being drawn from the solar array.

2. Subsystem Descriptions

FIG. 5 shows additional detail of an OCP circuit 500 for use in various embodiments of the present invention (for example, as circuit 282 in FIGS. 2, 3, and 25), including three over-current protection functions: (1) slow current limiting, (2) fast over-current regulation, and (3) prevention of current run-away due to unexpected negative rectified utility voltage V_(UT) that occurs as a result of loss of synchronization. For slow current-limiting function, the inverter RMS output current is controlled to not exceed its RMS reference limit, which is a function of the rectified utility voltage as determined at 581. The circuit block depicted as “limiting RMS current reference profile” 581 produces the RMS current limiting set-point, I_(Lim) _(—) _(ref), which commands the current-limiting error amplifier 589 to limit the output current to a corresponding RMS value. This slow current-limiting function preserves the sinusoidal waveform of the inverter output current since its control loop unity gain bandwidth is only 12 Hz to 15 Hz. The fast over-current shut-down circuit 587 provides an immediate reduction of the inverter output current when an over-current transient condition occurs that is considered unsafe to the inverter operation. The fast over-current regulator quickly limits the inverter output current to an instantaneous level corresponding to the fast current reference set-point I_(Lim) _(—) _(ref), which is determined by fast current command reference 583. The current level corresponding to I_(Lim) _(—) _(ref) can be considered as the upper ceiling limit that already above the sinusoidal steady state output current delivered during the slow current-limiting operation. Finally, a shut-down signal may be produced by negative voltage detection circuit 585 from the utility voltage V_(UT) when its negative voltage is present across the DC-DC converter output to prevent a run-away of the inverter output current. To effectively prevent the inverter current run-away, the 50/60 Hz switching bridge circuit 572 needs to be shut-down through the enable input of the 50/60 Hz bridge driver circuit 574 that is also shown in FIG. 8, below.

FIG. 6 reveals more detail of the OVP circuit 600, which provides two over-voltage protection functions: (1) slow voltage-limiting, and (2) fast over-voltage regulation. For slow over-voltage-limiting function, the inverter RMS output voltage is controlled to not exceed its RMS reference limit V_(RMS) _(—) _(ref), which is provided by voltage source 601. That signal commands the over-voltage-limiting error amplifier 602 to limit the output voltage to a corresponding RMS value. This slow over-voltage limiting function still preserves the sinusoidal waveform of the inverter output current since its control loop unity gain bandwidth is only 12 Hz to 15 Hz. The fast over-voltage shut-down circuit 604 provides an immediate reduction of the inverter output current when an over-voltage transient condition that is considered unsafe to the inverter operation occurs (relative to potential V_(Peak) _(—) _(ref) from voltage source 603).

In some embodiments, both current and voltage sensing schemes are implemented with electrical isolation from the converter output, though circuit detail for the isolated sensing of the converter output current I_(OUT) and output voltage V_(UT) is omitted from FIGS. 5 and 6. Given this direction, use of isolated sensing schemes is within the skill of the ordinary artisan.

FIG. 7 shows the 50/60 Hz switching bridge circuit 700 that performs commutation of the unidirectional output current I_(OUT) of the DC-DC converter in some embodiments of the present invention to get a bi-directional sinusoidal AC current for injecting into the utility grid through the utility grid interface circuit 710, which in some embodiments includes an output filter, a fuse, and an output relay-switching network. MOSFETs Q1 and Q4 are switched in complement with MOSFETs Q2 and Q3 with 50% duty cycle. A slightly overlapped on-time is allowed between these two pairs of MOSFETs to ensure that the souring current from the DC-DC converter output always observes low impedance during any commutation transition, thereby preventing harmful voltage spikes from happening during any commutation instant.

During a threat of an over-current run-away induced by an unexpected negative voltage appearing across the DC-DC converter output, the 50/60 Hz switching bridge may be shut down by the deactivated enable signal (see FIG. 8) that turns off the upper MOSFETs Q1 and Q3 to interrupt the inverter connection to the utility grid voltage (see FIG. 8 and accompanying discussion). During the severe circumstance of the switching bridge shut-down, the residual inductive kick-back energy from the DC-DC converter output is released through the transient absorption circuit consisting of D5, a full-bridge rectifier 720, and a set of transient absorption (e.g., TRANZORB) or over-voltage protection devices 730 as shown in FIG. 7.

Consequently, the 50/60 Hz switching bridge circuit 700 shown in FIG. 7 is universally applicable to any DC-DC converter topology that is capable of delivering continuous current from its output without needing to have a filtering capacitor across its output. For some converter topologies, it is still possible to have a DC-DC converter that delivers discontinuous current from its output and at the same time the unidirectional discontinuous current is injected into the switching bridge circuit for production of bi-directional discontinuous current supplying the utility grid via a properly designed output filter that is capable of removing most of the switching ripple out of trains of the pulsating current waveform and finally delivering continuous AC current to the utility grid.

FIG. 8 provides the detail of the 50/60 Hz bridge driver circuit 800 that feeds four driving signals to the four MOSFETs Q1 to Q4 in the switching bridge commutation network as shown in FIG. 7. The driving transformers T1 and T2 provide necessary electrical isolation among the four ports that deliver output driving signals (G1/S1, G2/S2, G3/S3, and G4/S4). Four sets of interconnected resistor and diode networks interfacing between the secondary winding outputs of the two transformers and the four driven gate-source terminal pairs ensure that, during every transition of inverter current commutation, acceptable but negligible overlapped on-time exists between the upper and lower MOSFETs in the switching bridge circuit.

A DC-DC converter power stage and a 50/60 Hz switching bridge circuit can be combined together to form subsystem 900 as shown in FIG. 9 to reduce the forward voltage drops of the power rectifiers. At the same time, these combined power circuits employ transformer T1 to provide electrical isolation from the upstream DC power system without losing the transient absorption capability. To significantly reduce loss from resistive current sensing schemes, and to provide isolation of the sensed current signal from the power circuit, a current-transformer sensing scheme is used instead through current-sense transformers T2 and T3 to obtain the isolated sensed current signal Isen for feedback control purposes. Coupled inductor L1 allows commutation of the unidirectional converter output current into the AC current supplying the utility grid interface circuit 910. At the same time, this design reduces the forward voltage drop to only two diodes, instead of the three-diode voltage drops found in many conventional single-winding inductors.

The preferred combined power and switching bridge circuit 900 contains an optimum number of active switches (six) for reliable and efficient power conversion, while retaining effective commutation of the converter output current. The same bridge driver circuit 800 shown in FIG. 8 is still applicable for controlling the four commutating power MOSFETs (Q1 to Q4) shown in FIG. 9. Only Q5 and Q6 are switched and pulse-width modulated at high frequency in tandem to regulate the converter output current in a rectified sinusoidal waveform. Diodes D2 and D4 protect against an inadvertent short-circuit across non-zero utility grid voltage since these two diodes prevent their respective power MOSFETs from conducting through their respective body diodes. Such a potentially catastrophic short-circuit could occur as a result of an inadvertent or temporary loss of grid-voltage synchronization of driving signals produced by the 50/60 Hz bridge driver circuit.

Another DC-DC converter power stage and a 50/60 Hz switching bridge circuit can be combined together to form subsystem 901 as shown in FIG. 24 to reduce the forward voltage drops of the power rectifiers. At the same time, these combined power circuits employ transformer T1 to provide electrical isolation from the upstream DC power system without losing the transient absorption capability. To significantly reduce loss occurring from resistive current sensing schemes, and to provide isolation of the sensed current signal from the power circuit, a current-transformer sensing scheme is preferably used instead. In this exemplary embodiment, current-sense transformer T3 obtains the isolated sensed current signal Isen for feedback control purposes. In subsystem 901, T1 serves two functions simultaneously: as energy storage through its magnetizing inductance, and as a transformer for immediate transfer of energy stored in capacitor C2 and energy from the DC input bus into inductor L and downstream circuitry.

Despite its highly pulsating wave shape, Isen has the same moving average value as the current through inductor L. Therefore, Isen can be controlled to have its moving average tracking a rectified sinusoidal wave shape while forward voltage drops of only two diodes are still achieved. The combined power and switching bridge circuit 901 also contains an optimum number of active switches (six) for reliable and efficient power conversion while retaining effective commutation of the converter output current.

The same bridge driver circuit shown in FIG. 8 is still applicable for controlling the four commutating power MOSFETs (Q1 to Q4) shown in FIG. 24. Only Q5 and Q6 are always switched in a complementary fashion (non-overlapping on-time between Q5 and Q6), while Q6 is pulse-width modulated at high frequency to regulate the converter output current in a rectified sinusoidal waveform. C1, Q5, and D6 provide an active voltage clamping mechanism to absorb parasitic energy stored in the leakage inductance of transformer T1, while at the same time releasing the excess transient energy to the output capacitor C2 through diode D5 during every on-time interval of Q5. D6 provides an immediate voltage clamping path due to residual current in the T1 leakage inductance at every turn-off interval of Q6; therefore, Q5 does not need to be exactly switched in a perfect complementary fashion with Q6. Diodes D2 and D4 provide an inadvertent short-circuit protection across non-zero utility grid voltage since these two diodes prevent their respective power MOSFETs Q2 and Q4 from conducting through their respective body diodes. In the same manner, diodes D1 and D3 prevent inadvertent conduction of the body diodes in MOSFETs Q1 and Q3, respectively.

FIG. 10 illustrates a power circuit 1000 that minimizes the forward voltage drops of the power rectifiers. This configuration consists of a push-pull DC-DC converter power stage and a 50/60 Hz unfolding circuit. At the same time, the combined power circuit 1000 employs a high-switching-frequency transformer T1 for providing electrical isolation from the upstream DC power system without losing the transient absorption capability. Center-taps are provided for both primary and secondary windings of transformer T1 that allow MOSFETs Q3 and Q4 to be switched in a symmetrical pulse-width modulated push-pull fashion and a full-wave rectification of the converter output current with only one forward diode voltage drop. The centered-tap secondary windings of transformer T1 and coupled inductor L1 allow commutation of the unidirectional converter output currents into continuous AC current supplying the utility grid interface circuit 1010 and at the same time further reducing the forward rectification voltage drop of only one diode. As a result, the dual centered-tap push-pull configuration produces one less diode voltage drop as compared to the combined power circuit configuration shown in FIG. 9. The combined power converter and 50/60 Hz unfolding network contains a minimal number of active switches (four switches) for reliable and efficient power conversion, while retaining effective commutation of the converter output current.

MOSFETs Q1 and Q2 of the unfolding network are switched in a complimentary fashion at the utility (50/60 Hz) frequency to alternately supply the converter's positive and negative currents to the utility grid. Q3 and Q4 are switched and pulse-width modulated at high frequency in an interleaving non-overlapped ON-time manner so as to regulate the converter output currents to be rectified sinusoidal waveform. Two sets of transient absorption circuits (one consisting of D7 and TZ1 and the other D8 and TZ2) provide transient energy absorption from transient over-voltages produced by external voltage surges from the utility grid or the residual energy stored in the coupled inductor L1 when both Q1 and Q2 are turned off to occasionally stop supplying the converter currents to the utility grid.

Electrical isolation between the inverter output and the remaining control circuits, including the solar array, the line-filter, the stand-by battery, and the converter input, is achievable through isolation schemes within the following subsystems shown in FIGS. 28 and 31: (1) isolated bi-directional inverter 143 through its internal isolation transformer, (2) PWM and gate drive circuit 300 through its isolated driving transformer or opto-isolated driving circuit that provides isolated driving signals to several semiconductor switches within the inverter 143, (3) utility voltage dropout detector 294 through its voltage-sense transformer for sensing the utility AC voltage V_(AC), (4) the sensed converter output current I_(OUT) through an isolated current sense scheme such as a current-sense transformer (FIG. 31), (5) ideal full-wave rectifier 302 (FIG. 28) for obtaining the rectified inverter output current signal I_(FB) through a current-sense transformer with an ideal full-wave rectification circuit, and (6) bridge rectifier 274 for providing the sensed inverter output voltage with full-wave rectification, V_(UT), through an isolated voltage sensing scheme such as a voltage-sense transformer and a bridge rectifier.

The system controller reference ground node or chassis ground as shown in FIGS. 2, 3, 25, 28, and 31 can be the power return terminal of the solar array to which all the sensed and isolated feedback and control signals are referenced. While the solar array and the line-filter may have dedicated transient energy absorption networks installed for differential-mode voltage transient absorption, the chassis ground being located at the solar array power return terminal provides the most effective path for absorption of the transient current induced by lightning at the inverter system input. Furthermore, the inverter output, the output filter 176, and the relay switching network 296 may be protected from harmful common-mode voltage transients induced by lightning coupled into the utility grid by having another transient energy absorption network installed at the utility grid output 280 with respect to the safety ground return terminal of the utility grid.

Several embodiments shown above possess unidirectional power flow capability that functions well with stand-alone resistive loads having insignificant inductive components, or with the near-ideal voltage sink offered by the utility grid. For highly inductive loads, the unidirectional converter topologies are less able to control their output current to track the sinusoidal waveform since their power stages cannot absorb the reactive energy that the inductive load produces twice in every utility period. For high-performance solar inverter systems that allow bi-directional flow and are capable of maintaining sinusoidal current waveforms while driving inductive loads, the inverter topology shown in FIG. 26 is preferred. That topology preserves input/output isolation and high switching frequency. With reference to FIG. 26, transistors Q5 and Q6 are always switched in a substantially perfect complimentary fashion with a 50% duty ratio. Transistors Q1 and Q3 are pulse-width modulated to control the inverter positive output current (I_(OUT)>0) while transistors Q2 and Q4 are turned off. When the inverter output is controlled to be negative (I_(OUT)<0), Q2 and Q4 are instead pulse-width modulated while Q1 and Q3 are turned off. Transient protection diodes TZ1 and TZ2 provide bi-directional transient energy absorption due to voltage surges and spikes from both the utility grid and excess energy stored in inductor L, regardless of the inverter mode of operation.

FIG. 27 provides typical waveforms of the gate-to-source driving signals for MOSFETs Q1 to Q6 for use in conjunction with the unidirectional control architecture shown in FIG. 28. In FIG. 27, X represents a square-wave signal of the inverter switching frequency (e.g., 100 KHz), and Y represents the controlling signal that commands the inverter power stage to drive either positive (Y=1) or negative (Y=0) sinusoid current I_(AC) into the utility grid or stand-alone load. Signal PWM represents a pulse-width modulated signal that is generated by the innermost current mode control loop that regulates I_(AC) to be sinusoidal.

Output signals V_(GSi), are functions of logical input signals X, Y, and PWM. In these embodiments: V _(GS1)=( X+X·PWM)·Y V _(GS2)=(X+ X ·PWM)· Y V _(GS3)=(X+ X ·PWM)·Y V _(GS4)=( X +X·PWM)· Y V_(GS5)= X V_(GS6)=X In other embodiments, various logical combinations may be used to control the inverter and power components according to the present invention as would occur to one of ordinary skill in the art.

FIG. 28 depicts an inverter control and power architecture that is applicable to inverter topologies as shown in FIGS. 26 and 29, for example, to achieve unidirectional power flow for which the gate-drive signals shown in FIG. 27 may be used. Note that the architecture of FIG. 28 actually provides bi-directional power flow capability, but the control is based on unidirectional feedback. Feedback of this nature makes it difficult to design a system capable of handling highly inductive utility grid or inductive stand-alone loads. V_(SIN) _(—) _(REC) is the reference rectified sinusoidal waveform that is produced by the free-running waveform generator 292. Because of the unidirectional characteristics of the V_(SIN) _(—) _(REC) signal, I_(REF) or the current reference commanding signal is unidirectional, and consequently the gate-drive signals shown in FIG. 27 are preferably applied to the MOSFET switches shown in FIGS. 26 and 29.

FIG. 29 is another variation of the inverter system shown in FIG. 26. The full-bridge MOSFET switch topology offers simpler construction of isolation transformer T1.

FIG. 30 provides typical waveforms of the gate-to-source driving signals for MOSFETs Q1 to Q6 for bi-directional power flow operation that allows the reactive energy component to flush back into the inverter. For controlling I_(AC)>0 (Y=1), gate-drive signals for Q1 and Q3 are always complimentary to each other based on an exclusive-OR logic for the PWM and X signals, while Q2 and Q4 are turned off. While controlling I_(AC)<0 (Y=0), gate-drive signals for Q2 and Q4 are always complimentary to each other based on an exclusive-OR logic for the PWM and X signals, while Q1 and Q3 are turned off. The equations governing the outputs in this embodiment are: V _(GS1)= (X⊕PWM)·Y V _(GS2)= (X⊕PWM)· Y V _(GS3)=(X⊕PWM)·Y V _(GS4)=(X⊕PWM)· Y V_(GS5)= X V_(GS6)=X

Other signaling patterns may be used in alternative embodiments, as would occur to those of ordinary skill in the art.

FIG. 31 shows an inverter control and power architecture that is applicable to inverter topologies shown in FIGS. 26 and 29 to achieve bi-directional power flow using the gate-drive signals shown in FIG. 30. The stand-by battery 214 and battery management control circuit 218 in FIG. 31 have the same function as described above in relation to FIG. 25, and the same modes of operation as shown in the Table. In this bi-directional power flow inverter system, the AC inverter output current, I_(OUT), is fed back in a bipolar fashion without going through a full-wave rectification to maintain its sinusoidal continuity in both its instantaneous value and its time derivative. The bi-directional waveform of the commanding reference current, I_(REF), is derived from the V_(SIN) _(—) _(AC) waveform being produced by a high-fidelity sinusoidal waveform generator that is either synchronized with the utility grid voltage (during the grid-tie operation) or free-running (during the stand-alone mode of operation).

The system control functions are very similar to those that exist in the unidirectional control and power architecture shown in FIG. 25. The bi-directional inverter control and power architecture provides significant improvement in the smooth regulation of the AC inverter output current despite the highly inductive impedance that sometimes exists in the long transmission line of the utility grid or a highly inductive stand-alone load. The “isolated bi-directional inverter” block shown in FIGS. 28 and 31 may have the same topology as shown in FIG. 26 or 29, where the transient protection diodes TZ1 and TZ2 are included for clamping both internal and external voltage transients to a safe level. MOSFET gate drive switching signals as shown in FIG. 30 are derived from two controlling signals: V_(CON), which is the signal output of the average current controller 144, and signal Y, which is the 50/60 Hz square-wave signal produced by the free-running generator 292. X is the high-switching-frequency square-wave signal that is internally generated within the PWM & Gate Drive circuit block. Similarly, signal PWM is internally generated with the PWM & Gate Drive circuit block, and is derived from the V_(CON) signal.

Furthermore, FIGS. 32 and 33 provide alternative replacements for the bi-directional power-flow inverter topologies of push-pull and full-bridge configurations as shown in FIGS. 26 and 29, respectively. Gate drive waveforms shown in FIG. 30 and the system control architecture shown in FIG. 31 are also applicable to these alternative inverter topologies. Since the body diodes within MOSFETs Q1 to Q4 have slow reverse recovery performance, the two alternative inverter topologies are less suitable for use in the unidirectional power flow control configuration. Therefore, the inverter architecture shown in FIG. 28 and gate drive waveforms shown in FIG. 27 are preferably not applied to the topologies shown in FIGS. 32 and 33.

For further improved efficiency in performance of the isolation transformer, its construction should to be as simple as possible. The inverter topology shown in FIG. 34 employs a simple transformer configuration while preserving input/output isolation and high switching frequency of operation. Two diagonal pairs of MOSFETs in the front-end switching bridge are switched in a complementary fashion with 50% duty ratio. Eight MOSFETs in the back-end switching bridge are pulse-width modulated. MOSFETs Q1, Q3, Q2B, and Q4B are pulse-width modulated to control the inverter positive output current (I_(OUT)>0) while MOSFETs Q1B, Q3B, and Q4 are turned off. When the inverter output current is controlled to be negative (I_(OUT)<0), MOSFETs Q1B, Q3B, Q2, and Q4 are instead pulse-width modulated, and MOSFETs Q1, Q2B, and Q4B are turned off. Transient protection diodes TZ1 and TZ2 provide bidirectional transient energy absorption due to voltage surges and spikes from both the utility grid and excess energy stored in inductor L, regardless of the mode of operation of the inverter. Instead of using the front-end switching bridge configuration as shown in FIG. 34, some embodiments of transformer T1 have a center-tap configuration with a push-pull front-end configuration with MOSFETs Q5 and Q6 as shown in FIG. 26. FIG. 35 provides typical waveforms of the gate-to-source driving signals for eight MOSFETs in the back-end switching bridge shown in FIG. 34 for use in conjunction with the unidirectional control architecture shown in FIG. 28.

FIG. 36 provides typical waveforms of the gate-to-source driving signals for eight MOSFETs Q1 to Q4 and Q1B to Q4B for a bi-directional power flow operation that allows the reactive energy component to smoothly flush back into the inverter. For controlling I_(OUT)>0 (Y=1), two pairs of gate drive signals for the Q1-Q3 pair and the Q2B-Q4B pair are always complementary to each other based on an exclusive-OR logic for the PWM and X signals while Q1B, Q3B, Q2, and Q4 are turned off. For controlling I_(OUT)<0 (Y=0), gate drive signals for the Q2-Q4 pair and the Q1B-Q3B pair are complementary to each other based on an exclusive-OR logic for the PWM and X signals, while Q1, Q3, Q2B, and Q4B are turned off.

Furthermore, FIG. 37 provides an alternative replacement for the bi-directional power-flow inverter topologies with front-end push-pull or front-end full-bridge configurations as shown in FIG. 34. Gate drive waveforms shown in FIG. 36 and the system control architecture shown in FIG. 31 are also applicable to the alternative inverter topology. Since the body diodes within the eight MOSFETs in the back-end switching bridge have slow reverse recovery performance, the alternative inverter topology is not suitable for use in the unidirectional power flow control configuration. Therefore, the inverter architecture shown in FIG. 28 and gate drive waveforms shown in FIG. 35 are preferably not applied in the topology shown in FIG. 37.

From the overall block diagram of the overall solar inverter system as shown in FIG. 1, the DC-DC converter power stage 140 including the average current regulation controller 144 forms a “current-mode” converter that accepts I_(REF) as its commanding current input and delivers the converter output current '_(OUT). The moving average of '_(OUT) is instantaneously regulated at I_(REF) as shown conceptually in FIG. 11.

In general, a different “current-mode” control approach can be employed instead of average current regulation control 144. At least three alternative “current-mode” control approaches are taken in various embodiments for tracking the sinusoidal commanding waveform I_(REF), including (1) peak-current programmed control, (2) hysteretic control, and (3) peak-current turn-off and zero-current-turn-on control. FIG. 12 illustrates peak-current programmed control, wherein instantaneous peaks of the converter output current are controlled to track I_(REF). FIG. 13 illustrates hysteretic current regulation control, in that both instantaneous peaks and troughs of the converter output current are respectively controlled to track the upper and lower commanding thresholds being generated from I_(REF) signal. For example, the upper and lower commanding threshold signals are respectively the peak and trough envelopes between which I_(REF) commanding signal always resides. FIG. 14 illustrates peak-current turn-off and zero-current turn-on control, wherein instantaneous peaks of the converter output current are controlled to track I_(REF) and troughs of the output current are controlled to reach zero current without zero-current dead time. As a consequence, the controller produces very high output current ripples having moving-average values that constitute a sinusoidal wave shape by proper output filtering.

One maximum power tracking (MPT) control circuit that is used with various embodiments of the present invention (for example, as MPT control in FIG. 1) is shown in FIG. 38, with partial schematics shown in FIGS. 39 and 40 and certain steady-state waveforms in FIGS. 41-43. The output voltage V_(SA) and current I_(SA) of solar array 110 are sensed with scaling factors k₁ and k₂, respectively, and are low-pass filtered by filters 3810 and 3820 to obtain signals V_(LP) and I_(LP) by removing switching noise. These two low-pass filters 3810 and 3820 may have the same or different corner frequencies, but preferably have corner frequencies of at least ten times their ripple frequency to preserve the phase of the sensed and filtered voltage and current signals V_(LP) and I_(LP)). Through an analog multiplier 3830, power signal P_(SA) is then derived from the product of V_(LP) and I_(LP). Both V_(LP) and P_(SA) signals are fed into their respective analog differentiators 3840 and 3850, which detect the time derivatives for both the voltage and power signals. The time derivative outputs Vac_(SA) and Pac_(SA) are preferably processed in block 3860 according to the maximum power tracking scheme described in U.S. Pat. Nos. 6,281,485 and 6,369,462. Therefore, it is both feasible and effective to implement MPT control circuit 120 (see FIG. 1) with simple analog and digital discrete parts without complex microprocessor-based circuitry. Application-specific integrated circuits (ASICs) will be developed in some embodiments using similar analog and discreet configurations as discussed herein and in the two patents just cited. Furthermore, when the solar array characteristics are weak due to long-term degradation and/or low solar flux, the inherent dither signals extracted from the solar array voltage and power signals could be too small to accurately determine a proper control direction toward the maximum power point. Under these circumstances, an additional dither signal may be injected and superimposed on the commanding set-point voltage V_(SP) to allow the extracted dither signals to achieve a better signal-to-noise ratio for better tracking efficiency as shown by an AC source 3570 in FIG. 44. The extra dither signal of a lower frequency with respect to the frequency of the inherent dither signal (100/120 Hz) is preferred, such as at 10 to 15 Hz, since the input voltage regulation control loop has the comparable unity-gain crossover frequency, thereby allowing the solar array voltage and current to produce sufficient AC amplitude of the extracted dither signals used for maximum power tracking.

As shown in FIGS. 39 and 40, two analog differentiator circuits are more simple to design and more economical to build than the double-stage sample-and-hold circuits with 2-phase clocks that are employed in the '485 and '462 patents cited in the paragraph immediately above. Waveforms of the 120/100 Hz input voltage and current ripples of the DC power sources are shown in the traces in the center strip of FIG. 41. These 120/100 Hz ripples inherently exist as a result of the rectified sinusoid current regulation that causes the converter to absorb the input current (see the top trace in FIG. 41) that is drawn from both the DC source and the bulk input capacitor C_(BULK). The peak current drawn by the converter is twice the average current drawn from the DC source. FIG. 41 illustrates the steady-state waveforms during the maximum power tracking mode, where the power drawn from the DC power source processes AC power ripple that has a frequency (240/200 Hz) twice the frequency of the AC voltage and current ripples superimposed on the DC source voltage and current. For a linear (resistive) DC source, the 240/200 Hz power ripple signal has a symmetric sinusoidal wave shape. For a non-linear DC source, the power ripple signal has an asymmetric sinusoidal wave shape as shown in the lower traces of FIG. 41.

FIGS. 42 and 43 show in more detail how the voltage and power ripple of a non-linear DC power source are overlaid. These figures illustrate power versus voltage characteristics of the non-linear DC power source for three operating conditions: (1) at the maximum power point (FIG. 42), (2) on the right side of the maximum power point on the characteristic curve (“out-of-phase” traces on the left side of FIG. 43), and (3) on the left side of the maximum power point along the characteristic curve (“in-phase” traces on the left side of FIG. 43).

The differentiator circuit of FIG. 39 is preferably designed to have a desired transfer function

$\frac{R_{L}}{R_{2}}{S \cdot C \cdot R_{1}}{\frac{\left( {{S\;\frac{L}{R_{L}}} + 1} \right)}{\left( {{S \cdot C \cdot R} + 1} \right)}.}$ When L/R_(L)=C·R, the circuit behaves as a differentiator with gain

$S \cdot C \cdot R_{1} \cdot {\frac{R_{4}}{R_{2}}.}$ Similarly, the alternative differentiator circuit in FIG. 40 has a transfer function

$\frac{R_{4}}{R_{2}}{S \cdot C \cdot R_{1}}{\frac{{S \cdot C_{2} \cdot R_{2}} + 1}{{S \cdot C \cdot R} + 1}.}$ When C·R=C₂·R₂, the circuit behaves as a differentiator with gain

$S \cdot C \cdot R_{1} \cdot {\frac{R_{4}}{R_{2}}.}$ 3. Simulation of the System

A computer simulation has been executed for the embodiment shown in FIG. 1, with the following results.

3.1 MPT of Mode I and IL

Under different weather conditions and solar radiation levels, the solar array 110 has different characteristic curves and different maximum power points. To demonstrate the adaptability of the inverter under the control of MPT controller, two modes of characteristic I-V curves are chosen in simulation, which are shown in FIG. 15. In this simulation, the two modes were switched linearly, and the transition time of these two modes was set to be 0.1 second. As shown in FIG. 15, the solar array jumps between these two modes as time passes. The maximum power for modes I and II are set at 600 watts and 480 watts, respectively.

As shown in FIG. 16, the power output of the solar array switches between 600 watts and 480 watts during solar array mode switching. This requires the output power of the solar array 110 to track the maximum power under the specific mode of the solar array 110 at that time. The simulation results show the stability of the system under the step change of the solar array maximum power point and the dynamic character of the MPT controller.

3.2 Utility Grid Interface.

FIG. 17 shows that the voltage and current of utility grid are 180° out of phase, so the energy from the solar array 110 will be charged into the utility grid at terminals 180. Further, the current is nearly pure sinusoid with fewer harmonics, so the utility grid will not be “polluted” by the inverter. As shown in FIG. 18, the amplitude of the solar array voltage ripple is very small even when the current injected into the utility grid is low-frequency sinusoid. After the maximum power point is achieved, there is still a 120/100 Hz ripple on the solar array voltage. The frequency of the solar array voltage ripple is double that of the utility AC power frequency, and the amplitude of the ripple is controlled in various embodiments by selection of capacitor C_(BULK) between the solar array and DC-DC converter. The second harmonic voltage ripple will worsen the performance of average current controller, and will increase the third harmonics on the utility grid current as discussed in L. H. Dixon, “High Power Factor Switching Preregulator Design Optimization,” Unitrode Power Supply Design Seminar Manual SEM800, 1991. To further minimize the solar array voltage ripple, an input filter is designed to dampen the specific frequency of 120/100 Hz. To overcome the problem caused by local maximum power point, a dither signal of lower frequency than 120/100 Hz (see Wenkai Wu, N. Pongratananukul, Q. Weihong, K. Rustom, T. Kasparis, I. Batarseh, “DSP-Based Multiple Peak Power Tracking for Expandable Power System,” Applied Power Electronics Conference and Exposition, APEC Eighteenth Annual IEEE, Feb. 9-13, 2003, Vol. 1, pages 525-530) will be applied in the MPT controller, so the solar array voltage will not be trapped at a local maximum power point that is not at the “true” peak power voltage.

3.3 DC-DC Converter.

To simplify the inverter design, the output current, 'OUT, and voltage, VUT, of DC-DC converter 142 are preferably synchronized rectified sinusoids as shown in FIG. 19. The output voltage of the DC-DC converter is just the rectified utility grid voltage ^(V)U1·'OUT will be controlled by I_(REF), the reference of average current controller, to yield synchronized rectified sinusoid output current as discussed in Jaber Abu-Qahouq and Issa Batarseh, “Unified Steady-State Analysis of Soft-Switching DC-DC Converters,” IEEE Transactions on Power Electronics, September 2002, Vol. 17, No. 5, pages 684-691.

3.4 Feedback Circuits.

The feedback portion of the proposed maximum power tracking solar inverter consists of the following blocks:

Average current controller 144 provides a band-limited control signal V_(CON) to the DC-DC converter to produce rectified sinusoid output current that is synchronized to the utility voltage. A reference current signal, I_(REF), generated by multiplier with feed forward compensation 152, decides the shape and amplitude of I_(OUT) as shown in FIG. 20.

Multiplier with feed-forward compensation 152 will generate I_(REF) from V_(UT) and V_(ERR). The signal V_(UT) is pre-processed before being sent to the multiplier 152, as discussed above.

Upon reaching steady state, SA voltage regulation error amplifier 130 keeps the solar array voltage V_(SA) regulated at the array maximum power voltage. V_(ERR), the output of SA error amplifier 130, determines the amplitude of I_(OUT). SA output power and voltage, with their corresponding control signals, are shown in FIGS. 21 and 22.

As shown in FIG. 21, V_(SP) is continually updated to settle at a voltage corresponding to the array peak-power voltage. When the maximum power point is achieved, V_(SP) goes to steady state to maintain the output power. Different steady states of V_(SP) correspond to different modes of solar array 110. Solar array voltage and output of SA voltage error amplifier (V_(SA) and V_(ERR)) are shown in FIG. 22.

3.5 Step Response of MPT.

To verify the step response of MPT controller 120, another simulation result is shown in FIG. 23, which shows the step response of SA power (I_(SA)·V_(SA)) and SA voltage (V_(SA)) when the RMS value of utility grid voltage jumps from 84V to 130V. This illustrates the stability of the system when the utility grid voltage changes.

All patents, publications, prior applications, and other documents cited herein are hereby incorporated by reference in their entirety as if each had been individually incorporated by reference and fully set forth.

While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected. 

1. A bi-directional power supply adapted to supply AC power to a utility grid or stand-alone load, comprising: a DC power source; a bi-directional inverter that provides output power through a single power stage of DC-AC conversion; an output filtering grid interface that filters and supplies the output power from the bi-directional inverter into the utility grid; and a system controller for regulating the voltage of the DC power source at its maximum power voltage, controlling the waveform of the inverter output current to be sinusoidal, and reducing the output power when the utility grid voltage exceeds a predetermined threshold; wherein the output power: tracks the maximum power output of the DC power source; and is sinusoidal AC current that is controlled to be other in phase with or optimally phase-shifted with the utility grid voltage; wherein the bi-directional inverter comprises a cascade connection of an input bulk capacitor for sufficient energy storage to limit voltage ripple across the DC power source to a predetermined level, a line-filter for filtering the inverter input current to suppress the conducted and radiated switching noise produced by the inverter, and the bi-directional inverter power stage; and wherein the output filtering grid interface comprises an output filter, a fuse, and an output relay switching network for transferring power to either the utility grid or the stand-alone load.
 2. The power supply of claim 1, further comprising: an input-output isolation means for electrically isolating the input of the bi-directional inverter from its output while electrical power is transferred from the inverter input to the inverter output through the isolation means operating at a switching frequency, which has an input and an output being electrically isolated from each other; a set of one or more controllable input semiconductor switches that are parts of the inverter input circuit but are electrically isolated from the inverter output and controlled by the system controller at the switching frequency to regulate the inverter output current to have a sinusoidal waveform that is either in phase with or optimally phase-shifted with the utility grid voltage; and an output filtering means for absorbing AC switching ripple current from the AC sinusoidal output current to produce a filtered AC sinusoidal output current, and transferring the filtered AC sinusoidal output current into the utility grid, and attenuating common-mode switching noise that radiates from the inverter semiconductor switching through the utility grid transmission line; wherein the input-output isolation means and the input semiconductor switches are parts of the bi-directional inverter and operate at the same switching frequency, which is at least about 20 kHz; wherein the inverter output has its power components internally interconnected from the output of the input-output isolation means to the inverter output that is externally terminated by the output filtering grid interface which comprises the output relay switching network and the output filter; wherein the inverter input includes the DC power source, the input bulk capacitor, the line filter, and all interconnected components that are electrically connected to the input of the input-output isolation means; and wherein the inverter semiconductor switching occurs at the same switching frequency and includes switching of all passive and active semiconductor switches within the inverter circuit.
 3. The power supply of claim 1, further comprising: an ideal full-wave rectifier configured to sense and convert the inverter output current into a rectified feedback current signal that is used to regulate the waveform of the inverter output current to be rectified sinusoidal; a rectified sinusoidal reference signal, V_(SIN) _(—) _(REC), that commands the rectified feedback current signal I_(FB) to track a reference current waveform I_(REF); an average current regulation controller provides an amplified frequency-compensated error signal, V_(con), from its two analog input signals, one being the rectified feedback current signal I_(FB) and the other being the reference current waveform, I_(REF); a uni-polar three-input multiplier and feed-forward compensation circuit that provides a reference current commanding signal, I_(REF)>0, from the rectified reference signal V_(SIN) _(—) _(REC), an error signal output of an input voltage regulation controller V_(ERR), and the inverse of the RMS value derived from a sensed utility grid voltage, V_(UT); a PWM and gate drive circuit that produces a set of isolated PWM switching signals that drive the controllable semiconductor switches within the bi-directional inverter power stage; wherein the PWM and gate drive circuit has two inputs, including one that receives the frequency-compensated error signal V_(con), and another that receives a sense signal, Y, that indicates the direction of current flow into or out of the utility grid for in-phase synchronization with the utility grid voltage.
 4. The power supply of claim 1, further comprising: a bipolar current sensor for sensing and converting the AC inverter output current into the bipolar feedback current signal I_(OUT) used for regulating its waveform to be sinusoidal; a sinusoidal reference signal, V_(SIN) _(—) _(AC), for providing the reference sinusoidal waveform that commands the bipolar feedback current signal I_(OUT) to track the reference waveform I_(REF); an average current regulation controller for providing an amplified frequency-compensated error signal, V_(con), from its two analog inputs, one is the said bipolar feedback current signal I_(FB) and the other the reference current commanding signal, I_(REF); a bipolar three-input multiplier and feed-forward compensation circuit for providing an appropriate reference current commanding signal, I_(REF), from the said reference sinusoidal signal V_(SIN) _(—) _(AC), the error signal output of the input voltage regulation controller V_(ERR), and the inverse of the RMS value derived from the sensed utility grid voltage, V_(UT); a PWM and gate drive circuit (block 300) for producing a set of isolated PWM switching signals that drive all the controllable semiconductor switches within the bi-directional inverter power stage; wherein the PWM and gate drive circuit have two inputs, one for the analog control signal V_(con) that is output from the average current regulation controller and the other, Y, for providing the direction of current flow into the utility grid for in-phase synchronization with the utility grid voltage.
 5. A bi-directional power supply adapted to supply AC power to a utility grid or stand-alone load, comprising: a DC power source; a bi-directional inverter that provides output power through a single power stage of DC-AC conversion; an output filtering grid interface that filters and supplies the output power from the bi-directional inverter into the utility grid; and a system controller for regulating the voltage of the DC power source at its maximum power voltage, controlling the waveform of the inverter output current to be sinusoidal, and reducing the output power when the utility grid voltage exceeds a predetermined threshold; the stand-by battery management control circuit controls a first and a second semiconductor battery switch to operate a stand-by battery in an isolation mode, a stand-by mode, a charging mode, an input voltage clamping mode, and a battery-dominated input mode; the stand-by battery is in the isolation mode when the first and second semiconductor battery switches are turned off; the stand-by battery is in the stand-by mode when the first semiconductor battery switch is turned on, the second semiconductor battery switch is turned off, and the inverter and output filtering grid interface provide transient power to the stand-alone load; the stand-by battery is in the charging mode when the first semiconductor battery switch is turned off and the second semiconductor battery switch is linearly regulated to maintain a healthy state of charge in the stand-by battery at low power, while the system controller regulates the DC power source output voltage at a level that is typically above the battery voltage; the stand-by battery is in the input voltage clamping mode when the first semiconductor battery switch is turned off, the second semiconductor switch is turned on continuously or linearly regulated with a pulse-width modulation control, and the inverter and output filtering grid interface are shut down, to restore a healthy state of charge to the stand-by battery without power transfer to the inverter; and the stand-by battery is in the battery-dominated input mode, when the first semiconductor battery switch is turned on and the second semiconductor switch is turned on continuously or linearly regulated with a pulse-width modulation control, for providing power transfer either to the stand-alone load when there is no power available from the DC power source, or for fast battery charging while the inverter is shut down; wherein the output power is controlled to track the maximum-power voltage of the DC power source and to have an output sinusoidal AC current that is controlled to be in phase with or optimally phase-shifted with the utility grid voltage when the DC power source is active and the stand-by battery management control circuit operates in any of the following modes: the battery-charging mode, the battery stand-by mode, the battery isolation mode; wherein the output power is controlled to have a regulated AC output voltage across the stand-alone load when the DC power source is inactive and the stand-by battery management control circuit operates in a battery-dominated input mode; wherein the output power is cut off from both the utility grid and the stand-alone load when the stand-by battery management control circuit operates in an input-voltage clamping mode for maintaining the stand-by battery; wherein the bi-directional inverter comprises a cascade connection of an input bulk capacitor for sufficient energy storage to limit voltage ripple across the DC power source to a predetermined level, a line-filter for filtering the inverter input current to suppress the conducted and radiated switching noise produced by the inverter, and the bi-directional inverter power stage; and wherein the output filtering grid interface comprises an output filter, a fuse, and an output relay switching network for transferring power to either the utility grid or the stand-alone load.
 6. The power supply of claim 1, wherein V_(ERR) serves as the commanding power input that is proportional to the average output power being transferred from the power converter to the utility grid and independent from the utility grid voltage; and further comprising a slow voltage-limiting error amplifier for providing active pull-down, through a first diode and an PNP transistor, of V_(ERR) when the RMS voltage across the stand-alone load or the RMS of the utility grid voltage exceeds a predetermined RMS value, while preserving the sinusoidal waveform of the AC inverter output current; a fast over-voltage shut-down circuit for providing a quick pull-down, through a second diode and the PNP transistor, of V_(ERR) to shut down the power converter stage when the voltage across the stand-alone load or the utility grid exceeds a predetermined instantaneous value; and an isolated voltage sense and scaling circuit that senses the rectified inverter output voltage with scaling factor K_(V) with electrical isolation from the inverter output for feeding the input of both the slow voltage-limiting amplifier and the fast over-voltage shut-down circuit.
 7. The power supply of claim 1, further comprising an over-current protection circuit for limiting the AC output current to a predetermined RMS value and preserving the sinusoidal waveform of the AC output current.
 8. The power supply of claim 1, further comprising: one or more power semiconductors in a component selected from the component group consisting of the bi-directional inverter and the output filtering grid interface; and an over-temperature protection circuit for sensing the temperature of the one or more semiconductors and regulating the sensed temperature not to exceed a predetermined temperature by reducing the output power transferred to the utility grid or a stand-alone load.
 9. The power supply of claim 1, further comprising: a transformer with two primary windings and two secondary windings, wherein the two primary windings are connected in series to provide an upper primary dotted terminal, a primary center tap terminal, and a lower primary un-dotted terminal, and wherein the two secondary windings are connected in series to provide an upper secondary dotted terminal, a secondary center-tap terminal, and a lower secondary un-dotted terminal; an output inductor having its negative terminal connected to the positive terminal of the output filtering grid interface and its positive terminal to a common node, to which the first set and the second set of unfolding unidirectional PWM switches are connected; a first set and a second set of unfolding unidirectional PWM switches, the first set directing positive current into the output filtering grid interface, and the second set for directing negative current into the utility grid; a first square-wave semiconductor switch having its DRAIN terminal connected to the upper primary dotted terminal of the transformer and its SOURCE terminal connected to the negative terminal of the DC input voltage; a second square-wave semiconductor switch having its DRAIN terminal connected to the lower primary un-dotted terminal of the transformer and its SOURCE terminal connected to the negative terminal of the DC input voltage; and a two-terminal bi-directional transient energy absorption circuit comprising two unidirectional transient energy absorption devices connected back-to-back in series, one of its terminals being connected to the common node and the other being connected to the secondary center-tap terminal of the transformer; wherein the first set of unfolding unidirectional PWM switches comprises an upper positive unfolding unidirectional switch and a lower positive unfolding unidirectional switch, the two positive unidirectional switches having their negative terminals tied together to the common node, the positive terminal of the upper positive unidirectional switch being connected to the upper secondary dotted terminal of the transformer, the positive terminal of the lower positive unidirectional switch being connected to the lower secondary undotted terminal of the transformer; wherein the second set of unfolding unidirectional PWM switches comprises an upper negative unfolding unidirectional switch and a lower negative unfolding unidirectional switch, the two negative unidirectional switches having their positive terminals tied together to the common node, the negative terminal of the upper negative unidirectional switch being connected to the upper secondary dotted terminal of the transformer, the negative terminal of the lower negative unidirectional switch being connected to the lower secondary undotted terminal of the transformer; wherein each of the four upper and lower unfolding unidirectional switches comprising a series connection of a diode and a semiconductor switch to form a unidirectional two-terminal switch having the series diode for enforcing unidirectional conduction of the corresponding semiconductor switch, wherein the primary center-tap terminal of the transformer is connected to the positive terminal of the DC input voltage; wherein the output filtering grid interface comprises the cascaded connection of an output filter, a fuse, and a relay switching network, the negative terminal of the output filtering grid interface being connected to the secondary center-tap terminal of the transformer and the positive terminal of the output filtering grid interface being connected to the negative terminal of the output inductor; and wherein the apparatus produces at most a two-semiconductor-switch voltage drop and a forward-bias voltage drop of only one diode at any switching state while the system is connected to the utility grid.
 10. The power supply of claim 1, further comprising: a transformer having one primary winding, a first secondary winding, and a second secondary winding, wherein the first and second secondary windings are connected in series to provide an upper secondary dotted terminal, a secondary center-tap terminal, and a lower secondary un-dotted terminal; a first set and a second set of unfolding unidirectional PWM switches, the first set directing positive current into the output filtering grid interface and the second set directing negative current into the utility grid; an output inductor having its negative terminal connected to the positive terminal of the output filtering grid interface and its positive terminal to a common node, to which the first set and the second set of unfolding PWM switches are connected; a front-end switching bridge comprising first, second, third, and fourth square-wave semiconductor switches, the first and second square-wave semiconductor switches having their DRAIN terminals connected to the positive terminal of the DC input voltage, the SOURCE terminal of the first square-wave semiconductor switch being connected to the positive output terminal of the switching bridge, and the SOURCE terminal of the second square-wave semiconductor switch being connected to the negative output terminal of the switching bridge, the third and fourth square-wave semiconductor switches having their SOURCE terminals connected to the negative terminal of the DC input voltage, the DRAIN terminal of the third square-wave semiconductor switch being connected to the positive output terminal of the switching bridge, and the DRAIN terminal of the fourth square-wave semiconductor switch being connected to the negative output terminal of the front-end switching bridge; and a two-terminal bi-directional transient energy absorption circuit comprising two unidirectional transient energy absorption devices connected back-to-back in series, one of its terminals being connected to the common node and the other being connected to the secondary center-tap terminal of the transformer; wherein the output filtering grid interface comprises a cascaded connection of an output filter, a fuse, and a relay switching network, the negative terminal of the output filtering grid interface being connected to the secondary center-tap terminal of the transformer and the positive terminal of the output filtering grid interface being connected to the negative terminal of the output inductor; wherein the first set of unfolding unidirectional PWM switches comprises an upper positive unfolding unidirectional switch and a lower positive unfolding unidirectional switch, the two positive unidirectional switches having their negative terminals tied together to the common node, the positive terminal of the upper positive unidirectional switch being connected to the upper secondary dotted terminal of the transformer, the positive terminal of the lower positive unidirectional switch being connected to the lower secondary undotted terminal of the transformer; wherein the second set of unfolding unidirectional PWM switches comprises an upper negative unfolding unidirectional switch and a lower negative unfolding unidirectional switch, the two negative unidirectional switches having their positive terminals tied together to the common node, the negative terminal of the upper negative unidirectional switch being connected to the upper secondary dotted terminal of the transformer, the negative terminal of the lower negative unidirectional switch being connected to the lower secondary undotted terminal of the transformer; wherein each of the four upper and lower unfolding unidirectional switches comprises a series connection of a diode and a semiconductor switch to form a unidirectional two-terminal switch having the series diode for enforcing unidirectional conduction of the corresponding semiconductor switch, wherein the primary dotted terminal of the transformer is connected to the positive output terminal of the front-end switching bridge, and the primary un-dotted terminal of the transformer is connected to the negative output terminal of the front-end switching bridge; and wherein the apparatus produces at most a three-semiconductor-switch voltage drop and a forward-bias voltage drop of only one diode at any switching state while the system is connected to the utility grid.
 11. The power supply of claim 1, further comprising: a transformer with two primary windings and two secondary windings, wherein the two primary windings are connected in series to provide an upper primary dotted terminal, a primary center tap terminal, and a lower primary un-dotted terminal, and wherein the two secondary windings are connected in series to provide an upper secondary dotted terminal, a secondary center-tap terminal, and a lower secondary un-dotted terminal; a first and a second unfolding bi-directional PWM switch, for directing positive and negative current into the utility grid; an output inductor that has their negative and positive terminals respectively connected to the positive terminal of the output filtering grid interface and a common node, to which the first and the second unfolding bi-directional PWM switches are connected; a first square-wave semiconductor switch having its DRAIN terminal connected to the upper primary dotted terminal of the transformer and its SOURCE terminal connected to the negative terminal of the DC input voltage; a second square-wave semiconductor switch having its DRAIN terminal connected to the lower primary un-dotted terminal of the transformer and its SOURCE terminal connected to the negative terminal of the DC input voltage; and a two-terminal bi-directional transient energy absorption circuit comprising two unidirectional transient energy absorption devices connected back-to-back in series, one of its terminals being connected to the common node and the other being connected to the secondary center-tap terminal of the transformer; wherein the output filtering grid interface comprises the cascaded connection of an output filter, a fuse, and a relay switching network, the negative terminal of the output filtering grid interface being connected to the secondary center-tap terminal of the transformer and the positive terminal of the output filtering grid interface being connected to the negative terminal of the output inductor; wherein the first unfolding bi-directional PWM switch comprises two semiconductor switches connected back-to-back in series forming a bi-directional two-terminal switch, wherein one of its two terminals is connected to the upper secondary dotted terminal of the transformer, and the other terminal is connected to the common node; wherein the second unfolding bi-directional PWM switch comprises two semiconductor switches connected back-to-back in series forming a bi-directional two-terminal switch, wherein one of its two terminals is connected to the lower secondary undotted terminal of the transformer, and the other terminal is connected to the common node wherein the primary center-tap terminal of the transformer is connected to the positive terminal of the DC input voltage; and wherein the apparatus produces at most a three-semiconductor-switch voltage drop and zero diode voltage drop at any switching state while the system is connected to the utility grid.
 12. The power supply of claim 1, further comprising: a transformer having one primary winding, a first secondary winding, and a second secondary winding, wherein the first and second secondary windings are connected in series to provide an upper secondary dotted terminal, a secondary center-tap terminal, and a lower secondary un-dotted terminal; a first and a second unfolding bi-directional PWM switch, for directing positive and negative current into the utility grid; an output inductor having its negative terminal connected to the positive terminal of the output filtering grid interface and its positive terminal to a common node, to which the first and the second unfolding bi-directional PWM switches are connected; a front-end switching bridge comprising first, second, third, and fourth square-wave semiconductor switches, the first and second square-wave semiconductor switches having their DRAIN terminals connected to the positive terminal of the DC input voltage, the SOURCE terminal of the first square-wave semiconductor switch being connected to the positive output terminal of the switching bridge, and the SOURCE terminal of the second square-wave semiconductor switch being connected to the negative output terminal of the switching bridge, the third and fourth square-wave semiconductor switches having their SOURCE terminals connected to the negative terminal of the DC input voltage, the DRAIN terminal of the third square-wave semiconductor switch being connected to the positive output terminal of the switching bridge, and the DRAIN terminal of the fourth square-wave semiconductor switch being connected to the negative output terminal of the front-end switching bridge; and a two-terminal bi-directional transient energy absorption circuit comprising two unidirectional transient energy absorption devices connected back-to-back in series, one of its terminals being connected to the common node and the other being connected to the secondary center-tap terminal of the transformer; wherein the output filtering grid interface comprises a cascaded connection of an output filter, a fuse, and a relay switching network, the negative terminal of the output filtering grid interface being connected to the secondary center-tap terminal of the transformer and the positive terminal of the output filtering grid interface being connected to the output inductor; wherein the first unfolding bi-directional PWM switch comprises two semiconductor switches connected back-to-back in series forming a bi-directional two-terminal switch, wherein one of its two terminals is connected to the upper secondary dotted terminal of the transformer, and the other terminal is connected to the common node; wherein the second unfolding bi-directional PWM switch comprises two semiconductor switches connected back-to-back in series forming a bi-directional two-terminal switch, wherein one of its two terminals is connected to the lower secondary undotted terminal of the transformer, and the other terminal is connected to the common node wherein the primary dotted terminal of the transformer is connected to the positive output terminal of the front-end switching bridge, and the primary un-dotted terminal of the transformer is connected to the negative output terminal of the front-end switching bridge; and wherein the apparatus produces at most a four-semiconductor-switch voltage drop and zero diode voltage drop at any switching state while the system is connected to the utility grid.
 13. The power supply of claim 1, further comprising: a transformer with a primary winding and a secondary winding; a back-end PWM switching bridge comprising: a first, a second, a third, and a fourth branch of unfolding bi-directional PWM switches, forming the four branches of the unfolding bi-directional PWM switches that are controlled to direct positive or negative current into the utility grid interface, a positive and negative input terminal serving as the input port of the back-end switching bridge, and a positive and a negative output terminal serving as the output port of the back-end switching bridge; an output inductor having its positive terminal connected to the positive output terminal of the back-end PWM switching bridge, and its negative terminal connected to the positive terminal of the output filtering grid interface; a front-end switching bridge comprising first, second, third, and fourth square-wave semiconductor switches, the first and second square-wave semiconductor switches being upper square-wave semiconductor switches that have their DRAIN terminals connected to the positive terminal of the DC input voltage, the SOURCE terminal of the first upper square-wave semiconductor switch being connected to the front-end positive output terminal of the switching bridge, and the SOURCE terminal of the second upper square-wave semiconductor switch being connected to the front-end negative output terminal of the switching bridge, the third and fourth square-wave semiconductor switches being lower square-wave semiconductor switches that have their SOURCE terminals connected to the negative terminal of the DC source, the DRAIN terminal of the third lower square-wave semiconductor switch being connected to the front-end positive output terminal, and the DRAIN terminal of the fourth lower square-wave semiconductor switch being connected to the front-end negative output terminal of the front-end switching bridge; and a two-terminal bi-directional transient energy absorption circuit comprising a first and a second unidirectional transient energy absorption device connected back-to-back in series, one of its terminals being connected to the positive output terminal of the back-end PWM switching bridge, and the other being connected to the negative output terminal of the back-end PWM switching bridge; wherein the output filtering grid interface comprises a cascaded connection of an output filter, a fuse, and a relay switching network, the negative terminal of the output filtering grid interface is connected to the negative output terminal of the back-end switching bridge, and the positive terminal of the output filtering grid interface is connected to the negative terminal of the output inductor; wherein each of the four branches of the unfolding bi-directional PWM switch of the back-end PWM switching bridge comprises two unidirectional PWM switches connected in parallel, each unidirectional PWM switch comprises a series connection of a diode and a semiconductor switch forming a unidirectional two-terminal switch having the series diode for enforcing unidirectional conduction of the corresponding semiconductor switch; wherein the two unidirectional switches are connected in parallel to form a branch of unfolding bi-directional PWM switch that allows two opposite directions of current flow, one direction through one unidirectional switch and the other direction through the other unidirectional switch; wherein the first and second branches of unfolding bi-directional PWM switch each has a first terminal connected to the dotted terminal of the secondary winding of the transformer, and the third and fourth branches of the unfolding bi-directional PWM switch each have a first terminal connected to the un-dotted terminal of the secondary winding of the transformer; wherein the second terminal of the first branch of unfolding bi-directional PWM switch is connected to the positive output terminal of the back-end PWM switching bridge, and the second terminal of the second branch of unfolding bi-directional PWM switch is connected to the negative output terminal of the back-end PWM switching bridge; wherein the second terminal of the third branch of unfolding bi-directional PWM switch is connected to the positive output terminal of the back-end PWM switching bridge, and the second terminal of the fourth branch of unfolding bi-directional PWM switch is connected to the negative output terminal of the back-end PWM switching bridge; wherein the dotted terminal of the primary winding of the transformer is connected to the positive output terminal of the front-end switching bridge, and the un-dotted terminal of the primary winding of the transformer is connected to the negative output terminal of the front-end switching bridge; and wherein the apparatus produces a four-semiconductor-switch voltage drop and a forward-bias voltage drop of only two diodes at any switching state while the system is connected to the utility grid.
 14. The power supply of claim 1, further comprising: a transformer with a primary winding and a secondary winding; a back-end PWM switching bridge, comprising: a first, a second, a third, and a fourth unfolding bi-directional PWM switch, the four unfolding bi-directional PWM switches being controlled to direct positive or negative current into the output filtering grid interface, a positive and negative input terminal serving as the input port of the back-end switching bridge, and a positive and a negative output terminal serving as the output port of the back-end switching bridge; an output inductor connected between the positive output terminal of the back-end PWM switching bridge and the positive terminal of the output filtering grid interface; a front-end switching bridge comprising first, second, third, and fourth square-wave semiconductor switches, the first and second square-wave semiconductor switches being upper square-wave semiconductor switches that have their DRAIN terminals connected to the positive terminal of the DC input voltage, the SOURCE terminal of the first upper square-wave semiconductor switch being connected to the front-end positive output terminal of the switching bridge, and the SOURCE terminal of the second upper square-wave semiconductor switch being connected to the front-end negative output terminal of the switching bridge, the third and fourth square-wave semiconductor switches being lower square-wave semiconductor switches that have their SOURCE terminals connected to the negative terminal of the DC source, the DRAIN terminal of the third lower square-wave semiconductor switch being connected to the front-end positive output terminal, and the DRAIN terminal of the fourth lower square-wave semiconductor switch being connected to the front-end negative output terminal of the front-end switching bridge; and a two-terminal bi-directional transient energy absorption circuit comprising a first and a second unidirectional transient energy absorption device connected back-to-back in series, one of its terminals being connected to the positive output terminal of the back-end PWM switching bridge, and the other being connected to the negative output terminal of the back-end PWM switching bridge; wherein the output filtering grid interface comprises a cascaded connection of an output filter, a fuse, and a relay switching network, the negative terminal of the output filtering grid interface is connected to the negative output terminal of the back-end switching bridge, and the positive terminal of the output filtering interface is connected to the output inductor; wherein each of the four unfolding bi-directional PWM switches of the back-end PWM switching bridge comprises two semiconductor switches connected back-to-back in series; wherein the first and second unfolding bi-directional PWM switches each have a first terminal connected to the dotted terminal of the secondary winding of the transformer, and the third and fourth unfolding bi-directional PWM switches each have a first terminal connected to the un-dotted terminal of the secondary winding of the transformer; wherein the second terminal of the first unfolding bi-directional PWM switch is connected to the positive output terminal of the back-end PWM switching bridge, and the second terminal of the second unfolding bi-directional PWM switch is connected to the negative output terminal of the back-end PWM switching bridge; wherein the second terminal of the third unfolding bi-directional PWM switch is connected to the positive output terminal of the back-end PWM switching bridge, and the second terminal of the fourth unfolding bi-directional PWM switch is connected to the negative output terminal of the back-end PWM switching bridge; wherein the dotted terminal of the primary winding of the transformer is connected to the positive output terminal of the front-end switching bridge, and the un-dotted terminal of the primary winding of the transformer is connected to the negative output terminal of the front-end switching bridge; and wherein power flows from the DC source to the utility grid interface with active switch conduction loss of not more than a six-semiconductor-switch voltage drop, and the apparatus produces a zero-diode conduction loss or zero-diode forward-bias voltage drop at any switching state while the system is connected to the utility grid.
 15. The power supply of claim 10, wherein the two square-wave semiconductor switches and the two sets of unfolding unidirectional PWM switches in the bi-directional inverter have their on and off states of switching as follows: the two square-wave semiconductor switches, Q5 and Q6, have complimentary on and off states of switching as a function of their respective switching drive signals, V_(GS5) and V_(GS6), which are functions of a square-wave signal, X, at a fixed switching frequency as V_(GS5)= X and V_(GS6)=X. the four semiconductor switches being distributed in the two sets of unfolding unidirectional PWM switches, Q1 to Q4, have their on and off switching states in a pulse-width modulated (PWM) fashion through their respective drive signals, V_(GS1), V_(GS2), V_(GS3), and V_(GS4), which are functions of a PWM signal, the square-wave signal X, and the output current direction signal Y, satisfying the following logical relations: V _(GS1)=( X +X·PWM)·Y, V _(GS2)=(X+ X ·PWM)· Y , V _(GS3)=(X+ X ·PWM)·Y, and V _(GS4)=( X +X·PWM)· Y ; and wherein the PWM signal has its on and off switching state repeated at twice the switching frequency of signal X, and its on-state pulse-width is controlled by an analog signal V_(con) of the system controller mentioned in claim 4 and V_(con) is the output of the average current regulation controller within the system controller; wherein the output current direction signal Y represents the direction of the output current that has two logical states for controlling the output current to have a desired direction, one state for a positive direction and the other state for a negative direction, and is produced from the free-running waveform generator that is not synchronized with the utility grid voltage during a stand-alone mode, but is synchronized and in-phase with the utility grid voltage during a grid-tie mode of operation.
 16. The power supply of claim 12, wherein the square-wave semiconductor switches and the two sets of unfolding bi-directional PWM switches have their on and off states of switching as follows: the two square-wave semiconductor switches, Q5 and Q6, have complementary on and off switch states being controlled by their respective complementary switching drive signals, V_(GS5) and V_(GS6), which are functions of a square-wave signal, X, at a fixed switching frequency as V_(GS5)= X and V_(GS6)=X; the four unfolding bi-directional PWM semiconductor switches, Q1 to Q4, have pulse-width modulated (PWM) on and off switching states as a function of their respective drive signals, V_(GS1), V_(GS2), V_(GS3), and V_(GS4), which are functions of a PWM signal, the X signal, and the output current direction signal Y, satisfying the following logical relations: V _(GS1)= (X⊕PWM)·Y, V _(GS2)= (X⊕PWM)· Y, V _(GS3)=(X⊕PWM)·Y, and V _(GS4)=(X⊕PWM)· Y; wherein the PWM signal has its on and off switching state repeated at twice the switching frequency of signal X, and its on-state pulse-width is varied as a function of the analog signal V_(con) that is the output of the average current regulation controller mentioned in claim 4; wherein the Y signal is represents the direction of the output current that has two logical states for controlling the output current to have a desired direction, one state for a positive direction and the other state for a negative direction, and is produced from the free-running waveform generator that is not synchronized with the utility grid voltage during a stand-alone mode, but synchronized and in-phase with the utility grid voltage during a grid-tie mode of operation.
 17. The power supply of claim 12, wherein the square-wave switches and the two sets of unfolding bi-directional PWM semiconductor switches have their on and off states of switching as follows: the two square-wave semiconductor switches, Q5 and Q6, have complementary on and off switch states being controlled by their respective complementary switching drive signals, V_(GS5) and V_(GS6), which are functions of a square-wave signal, X, at a fixed switching frequency as V_(GS5)= X and V_(GS6)=X; the four unfolding bi-directional PWM semiconductor switches, Q1 to Q4, have pulse-width modulated (PWM) on and off switching states as a function of their respective drive signals, V_(GS1), V_(GS2), V_(GS3), and V_(GS4), which are functions of a PWM signal and the X signal, satisfying the following logical relations: V _(GS1) =V _(GS2)= (X⊕PWM), and V _(GS3) =V _(GS4)=(X⊕PWM); wherein the above logical relations for the drive signals V_(GS1) to V_(GS4) for controlling the four unfolding bi-directional PWM semiconductor switches Q1 to Q4 are also used for controlling the four unfolding unidirectional PWM semiconductor switches; and wherein the above logical relations for the drive signals V_(GS1) to V_(GS4) for the four bi-directional switches Q1 to Q4 are also applied to the following eight drive signals of the eight bi-directional switches Q1 to Q4 and Q1B to Q4B distributedly belonging to the four unfolding PWM switches of the back-end PWM switching bridge as follows: V _(GS1) =V _(GS1B) =V _(GS3) =V _(GS3B)= (X⊕PWM), and V _(GS2) =V _(GS2B) =V _(GS4) =V _(GS4B)=(X⊕PWM).
 18. The power supply of claim 1, further comprising an over-current protection circuit for limiting the AC output current to a predetermined RMS value and preserving the sinusoidal waveform of the AC output current, and the system controller includes the average current regulation error amplifier circuit for producing their respective control analog signal, V_(con) that is converted, through a pulse-width modulation (PWM) comparator and logic circuitry, into appropriate switching signals used for driving the semiconductor switches within the inverter power stage; wherein the average current regulation error amplifier and the subsequent PWM comparator and logic circuitry are replaced with different regulation circuits that employ one of the following control schemes: (1) the peak current-programmed control, (2) the hysteretic current regulation, and (3) the zero-current-turn-on and peak current turn-off control; wherein the said peak current-programmed control is a means for regulating the peak of the inverter output current to track the sinusoidal envelope of the reference sinusoidal waveform; wherein the said hysteretic current regulation is a means for regulating the peak and the trough of the inverter output current to track the two values corresponding to the upper and lower threshold values between which the sinusoidal reference current commanding signal resides in; and wherein the said zero-current turn-on and peak-current turn-off control is a means for regulating the peak of the inverter output current to track the sinusoidal envelope of the reference sinusoidal waveform and the trough of the inverter output current at zero current without zero-current dead-time. 